Purification of silver halide precipitates

ABSTRACT

AN IMPROVED METHOD FOR WASHING SILVER HALIDE PRECIPITATES BY EMPLOYING ION-EXCHANGE RESINS WHICH ARE CAPABLE OF SUBSTITUTING HYDROXYL IONS AND HYDROGEN IONS FOR EXTRANEOUS SOLUBLE SALTS.

United States Patent 3,782,953 PURIFICATION OF SILVER HALIDE PRECIPITA'IES Stephen Barry Maley, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, NY. No Drawing. Filed Sept. 20, 1971, Ser. No. 182,179 Int. Cl. G03c 1/02, 1/04 U.S. Cl. 96-94 R Claims ABSTRACT OF THE DISCLOSURE An improved method for washing silver halide precipitates by employing ion-exchange resins which are capable of substituting hydroxyl ions and hydrogen ions for extraneous soluble salts.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to photographic materials and their preparation.

The invention especially relates to a process for the preparation of photographic silver salt emulsions in which the extraneous soluble salts are removed by contacting the emulsion with ion-exchange resins. The invention additionally relates to silver halide emulsions precipitated in a polymeric vehicle in which the extraneous soluble salts are removed by ion-exchange resins.

Description of the prior art In the formation of silver halides by precipitation, in the presence of a peptizer, from a water soluble silver salt such as, for example silver nitrate, and a Group I halide solutions, soluble salts, generally nitrates, are also produced.

In order to obtain high quality photographic products it is generally necessary to remove the soluble salts from the emulsion. The extraneous salts have been removed after precipitation by various means, all well known in the art, such as by noodle washing, coagulation and decantation washing. The prior art methods of washing silver halide emulsions are necessarily time-consuming procedures, since it is necessary to set the emulsion for relatively long periods of time or to coagulate the emulsion prior to decantation. The processes involve the use of large volumes of clear, pure water which water is polluted by the washing process. Additionally, while gelatin has generally been the most often employed peptizer for photographic emulsions there are certain disadvantages inherent in gelatin such as, for example, susceptibility to attack by bacteria and difliculty of control of uniformity of gelatin compositions used as peptizers. In order to eliminate the problems inherent with gelatin there has been a continuing program to develop synthetic peptizers. Unfortunately, many silver halide emulsions prepared in synthetic peptizers are not capable of being washed by the procedures described in the prior art and generally therefore the peptizer composition requires the addition of gelatin in order to make washing possible.

The method of precipitating a photographic silver halide emulsion in the presence of an ion-exchange resin is described by H. D. Porter in U.S. Pat. No. 2,678,885. The Porter disclosure, however does not disclose washing and purifying silver halide emulsion upon precipitation in the presence of a peptizer.

The process as described by Porter does not generally facilitate pAg control and the use of ripening agents, dopants and the like during the precipitation step.

It is, therefore, desirable that an improved method be discovered for removing extraneous water soluble salts from silver halide emulsions precipitated in the presence 3,782,953- Patented Jan. 1, 1974 of a peptizer without detrimentally affecting the sensitometric properties of the photographic emulsions and without polluting large volumes of clear, pure water.

SUMMARY OF THE INVENTION In accordance with this invention there is provided an improved photographic process comprising contacting a silver salt emulsion, preferably a silver halide emulsion, with ion-exchange resins in order to remove extraneous soluble salts. The technique is suitable for washing emulsions precipitated in the presence of gelatin, gelatin derivatives or synthetic polymeric peptizers.

It is an object of this invention to provide a new method of washing photographic silver halide emulsions prepared in the presence of a peptizer. Another object is to provide a new method of washing precipitated silver halide emulsions prepared in the presence of a peptizer which generally cannot be Washed by the conventional prior art techniques.

It is yet another object of the invention to provide an improved method of washing silver halide emulsions prepared in the presence of a synthetic polymeric peptizer without adding gelatin to the enmlsion.

It is still another object of this invention to provide a method of washing silver halide emulsions which is generally more rapid and efficient compared with the known conventional washing techniques.

It is still another object of the invention to provide a method of washing photographic emulsions without employing water washing techniques thereby eliminating the pollution of clear, pure water.

It is still another object of the invention to provide a fast and efiicient method of washing freshly formed silver halide emulsion wherein the silver halide has been formed in the presence of ripening agents, dopants and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is possible, according to the present invention, to remove substantially all extraneous soluble salts from a silver salt emulsion comprising insoluble silver salts, extraneous soluble salts, peptizer and solvent by contacting the emulsion with ion exchange materials.

A preferred embodiment relates to the preparation of photosensitive silver halides. The process comprises precipitating, in the presence of a peptizer, silver halides from a solution of a soluble silver salt and a solution of one or more soluble halide salts, and removing extraneous soluble salts from the emulsion by contacting the so formed emulsion with ion-exchange resins.

In another preferred embodiment, the photosensitive silver halides are formed in the presence of a synthetic polymeric peptizer, thereby providing an emulsion which heretofore was generally not adaptable to washing by the conventional Washing techniques, and washing said emulsion by contacting the same with ion exchange resins.

In accordance with the invention the unwashed emulsion is contacted with one or more suitable ion-exchange resins. The ion-exchange resins should be capable of exchanging the by-product extraneous soluble salts for hydrogen and hydroxyl ions. The exchange can be accomplished in a batch manner by adding the combination of a cationic exchange resin and an anionic exchange resin directly to the emulsion with stirring followed by filtra tion in order to remove the resin beads. The exchange can readily be accomplished by one or more passages of the silver halide emulsion through a conventional ion-exchange column containing a mixture of anionic and cationic exchange resins (monobed). The exchange can also be readily accomplished by sequential passage of the emu1- sion through separate columns of cationic and then anionic exchange resins, or vice versa, to remove soluble ions without mixing the resins, thus facilitating regeneration of the columns for repeated usage. The procedure can be modified by running the emulsion through a series of several short columns of alternate type exchange resins to achieve a more gradual removal of ions thereby reducing pH variation of the emulsion.

In the case of emulsions of concentrations greater than 3 percent gelatin, it may be necessary to use a warm Water jacketed column. The warm water can vary over a wide range of temperatures depending on the concentration of and the type peptizer vehicle. In general it has been found that a temperature of from about 35 C. to about 40 C. is suitable. Lower gelatin concentrations need only have the column prewarmed by running through some hot water (40-50 C.) prior to starting. Non-gelling protective colloids can be run at normal ambient temperatures or even lower temperatures.

Any number of cationic and anionic resins can be employed in the process of this invention. The only limitation upon the resins is that they should be capable of exchanging hydrogen and hydroxyl ions for the undesirable, soluble salts formed as by-product of the silver halide precipitation without detrimentally affecting the sensitometric properties of the emulsion. It is generally preferable to employ strongly acidic and strongly basic resin exchangers, however under some situations it can be desirable to employ weakly acidic and weakly basic ion-exchangers. Typical situations which could require weak ion-exchangers are: the need for selective removal of specific ions from the emulsion, the sensitivity of some peptizers to the strong ion-exchangers and the like.

The resins which can be employed in accordance with the invention are well known. Typically the resins comprise a matrix and an active group. Illustrative resins are: polystyrene containing as an active group one of trirnethyl benzyl ammonium, quaternary ammonium, dimethyl ethanol benzyl ammonium; epoxy polyamines containing as an active group one of tertiary amine, quaternary ammonium; phenolic resins containing as the active group one of tertiary amine or secondary amine; polystyrene containing as the active group nuclear sulfonic acid or phosphonic acid and; phenolic resins containing as the active group one of methylene sulfonic or carboxylic.

The methods of regeneration of the resins are Well known and involve such means as regenerating a cationic resin with a mineral acid such as hydrochloric acid, sulfuric acid and the like and regenerating an anionic resin with a base such as sodium hydroxide and the like.

The type resins employed and the number of passes of an emulsion through a column is generally a function of the concentration of extraneous salts to be removed. The complete removal of all extraneous salt in some instances is to be avoided since a residual amount of by-product extraneous soluble salts help to prevent clumping of the emulsion. Hence in a preferred embodiment of this invention a photographic silver halide emulsion is passed through an ion-exchange column until about 97.3% of the extraneous salts are removed or until a pAg of 5.5- 8.5 is reached.

The silver halide emulsions prepared in accordance with this invention can comprise for example, silver chloride, silver bromide, silver bromoiodide, silver chlorobromide, silver chloroiodide, silver chlorobromoiodide crystals or mixtures thereof. The emulsions may be coarse or fine grain emulsions and prepared by any of the well-known techniques. Typical procedures for such preparation include single-jet procedures, double-jet procedures, procedures utilizing automatic proportional control means to maintain specified pAg and pH, procedures using ripening agents such as thiocyanates, thioethers and/or ammonia, procedures utilizing an increase in flow rates as disclosed in Wilgus, U.S. Ser. No. 244,230 filed Apr. 14, 1972 which is a continuation-in-part of U.S. Ser. No. 11,838 filed Feb.

16, 1970, now abandoned, hot nucleation procedures as disclosed in Musliner, U.S. Ser. No. 236,219 filed Mar. 20, 1972 which is a division of U.S. Ser. No. 31,351 filed Apr. 23, 1970, now abandoned, and the like. Surfaceimage emulsions may be prepared or internal-image emulsions such as those described in U.S. Pats. 2,592,250 by Davey et al., 3,206,313 by Porter et al., 3,367,778 by Berriman and 3,447,927 by Bacon et al. The silver halide grains can have metal dopants or metal ions occluded therein.

The silver salts to be washed in accordance with this invention can be precipitated in the presence of a peptizer, surfactant or other precipitation aids. The peptizers are generally added in order to prevent undesirable physical or chemical effects on the grain such as clumping and the like. The precipitation aids or peptizers can be introduced with the respective soluble silver salt and halide salt, or they can be present in the precipitation vessel before precipitation is initiated. In general, the precipitation of photographic silver halides is carried out in the presence of peptizers such as gelatin or synthetic polymeric peptizers and hydrophilic polymers as disclosed in Perry et al., U.S. Pat. 3,425,836 issued Feb. 4, 1969, acrylyl or methacrylyl histidine polymers such as disclosed in U.S. Pat. 3,419,397 issued Dec. 31, 1968, hydrophilic polymers such as disclosed in Whiteley et al., U.S. Pat. 3,392,025, interpolymers containing vinylamine units as disclosed in Smith et al., U.S. Pat. 3,415,653 issued Dec. 10, 1968, interpolymers such as disclosed in Hollister, U.S. Pat. 3,536,677 issued Oct. 27, 1970 interpolymers as disclosed in Smith et al., U.S. Pat. 3,615,624 issued Oct. 26, 1971 and the like. In accordance with this invention, and as indicated herein above, it is especially desirable to wash silver halides which have been precipitated in the presence of synthetic peptizers which cannot be washed by procedures described in the prior art, such as noodle washing or gelatin coagulation Washing.

The silver halides and other photographic silver salts washed in accordance with this invention may be dispersed in a variety of vehicles. The vehicles employed can be the same as or different from the peptizer employed in the precipitation. Suitable hydrophilic materials include both naturally-occurring substances such as proteins, for example, gelatin, gelatin derivatives, cellulose derivatives, polysaccharides such as dextran, gum arabic and the like; and synthetic polymeric substances such as water soluble polyvinyl compounds, e.g., poly(vinylpyrrolidone), acrylamide polymers and the like.

The photographic emulsion layers and other layers prepared in accordance with the practice of this invention can also contain alone or in combination with hydrophilic, Water permeable colloids, other synthetic polymeric compounds such as dispersed vinyl compounds such as in latex form and particularly those which increase the dimensional stability of the photographic materials. Suitable synthetic polymers include those described, for example, in Nottorf U.S. Pat. 3,142,568 issued July 28, 1964; White U.S. Pat. 3,193,386 issued July 6, 1965; Houck et al. U.S. Pat. 3,062,674 issued Nov. 6, 1962; Houck et al. U.S. Pat. 3,220,844 issued Nov. 30, 1965; Ream et al. U.S. Pat. 3,287,289 issued Nov. 22, 1966; and Dykstra U.S. Pat. 3,411,911 issued Nov. 19, 1968; particularly effective are those water-insoluble polymers of alkyl acrylates and methylacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates, those which have crosslinking sites which facilitate hardening or curing, those having recurring sulfobetaine units as described in Dykstra Canadian Pat. 774,054.

The photographic emulsions and layers prepared in accordance with the inventions described herein may be coated on a wide variety of supports. Typical supports include cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film and related films or resinous materials, as well as glass, paper, metal and the like.

Typically, a flexible support is employed, especially a paper support, which can be partially acetylated or coated with baryta and/or an alpha-olefin polymer, particularly a polymer of an alpha-olefin containing 2 to 10 carbon atoms such as polyethylene, polypropylene, ethylenebutene copolymers and the like. I

The photographic layers prepared in accordance with the practice of this invention may contain surfactants such as saponin; anionic compounds such as the alkyl aryl sulfonates described in Baldsiefen US. Pat. 2,600,831 issued June 17, 1952; amphoteric compounds such as those described in Ben-Ezra US. Pat. 3,133,816 issued May 19, 1964; and water soluble adducts of glycidol and an alkyl phenol such as those described in Olm Mathieson British Pat. 1,022,878 issued Mar. 16, 1966.

The photographic layers and emulsions prepared 1n accordance with the practice of this invention may be coated by various coating procedures including dip coating, air knife coating, curtain coating, or extrusion coating using hoppers of the type described in Begum U.S. Pat. 2,681,294 issued June 15, 1954. If desired, two or more layers may be coated simultaneously by the procedures described in Russell US. Pat. 2,761,791 issued Sept. 4, 1956; Huges U.S. Pat. 3,508,947 issued Apr. 28, 1970; and Wynn British Pat. 837,095 issued June 9, 1960. This invention also can be used for silver halide layers coated by vacuum evaporation as described in British Pat. 968,- 453 issued Sept. 2, 1964 and Lu Valle et al. US. Pat. 3,219,451 issued Nov. 23, 1965.

The photographic melts and emulsions produced in accordance with the practice of this invention may contain the normal addenda useful in photographic products such as photographic silver halides. The addenda can be added to the emulsion prior to coating, at the completion of the precipitation or during the final stages of the precipitation. Typical addenda which may be added are chemical sensitizers, development modifiers, antifoggants and stabilizers, developing agents, hardeners, spectral sensitizers and the like.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A radiation-sensitive gelatino-silver chrorobromide mole percent chloride and 95 mole percent bromide) photographic emulsion is prepared by slowly adding simultaneously an aqueous solution of silver nitrate and an aqueous solution of alkali metal halides to an agitated 4.2 percent by weight solution of copoly(3-thiapentylacrylate-acrylic acid 3 acryloxypropane-l-sulfonic acid, sodium salt), which contains 7.5 mg. of bismuth trinitrate pentahydrate per silver mole, at 30 C. and a pH of about 2.0.

At the end of the run (pAg 9.2), the emulsion is divided in two equal portions. Gelatin 190 gms./Ag mole) is added to the first portion. The emulsion is chill set and noodle washed in the conventional manner for two hours to a pAg 7.5. The second portion is washed in order to remove the extraneous soluble salts by adding with constant stirring, Amberlite MB-1 (a trademark of the Rohm and Haas Company of Philadelphia, Pa.) mixture of cationic and anionic exchange resins. After five minutes of stirring, the emulsion is at pAg 5.8 and is filtered to remove the ion-exchange beads and gelatin is added. Both emulsions are coated on a photographic paper support at about 60 mg. of silver/ft. and 550 mg. of dry gelatin/ ft.

Samples of each coating are exposed for one second through a 0.15 density increment step wedge, then placed in contact with a heated platen at 235 C. for 5 seconds, and photodeveloped for 5 minutes with two 8 watt BL fiourescent lamps.

A comparison of the two coatings shows essentially identical photographic properties.

6 EXAMPLE 2 A large grain (about 1.0 micron edge length) silver bromoiodide emulsion (2.5 mole percent iodide) is prepared by adding an aqueous solution of silver nitrate to a rapidly agitated gelatin solution'containing potassium bromide and potassium iodide. After precipitation the emulsion, having a K+ concentration of 16.1 mg./ml., is divided into two equal portions (identified as A and B) and washed by the following separate procedures:

A is noodle washed by adding gelatin, chilling, shredding and washing for 3 hours to obtain a pAg 8.2. The K+ ion concentration is 1.43 mg./ml.

B is washed by the ion-exchange procedure as described in Example 1. After 15 minutes of stirring, the emulsion is filtered and the pAg is measured at 7.7. The K+ concentration is 0.89 mg./ml.

The washed emulsion samples are adjusted to a pAg 8.4, chemically sensitized, exposed and coated on a film support. The results, after processing, are summarized in Table 'I.

TABLE I Relative Emulsion Wash method speed 7 Dan... Dmx.

A Noodle wash 100 94 06 1. B Ion-exchange resins 110 .95 .08 1.80

EXAMPLE 3 A cubic bromoiodide emulsion (0.2 micron) is prepared by adding an aqueous solution containing potassium bromide and potassium iodide and an aqueous solution of silver nitrate to a rapidly agitated gelatin solution. After precipitation, the emulsion is divided into separate portions, identified as C, D and E. Portions C and D are noodle washed for three hours and portion E is washed in accordance with the practice of this invention. The emulsions are then adjusted to a pAg 8.4, chemically sensitized and finished at the times indicated in the following table, coated, exposed and processed. The results are summarized in Table II.

TABLE II Fin- Rela- Ernulish tive slon Wash method time speed 7 Drain max C Noodle wash 30' 100 2.17 .04 2. 0 D d0 55 141 1. 97 04 2. 0 E Ion exchange resins 25 141 2.07 .04 2.1

EXAMPLE 4 A radiation-sensitive gelatino-silver chlorobromide (5 mole percent chloride and mole percent bromide) photographic emulsion is prepared as in Example 1 with the exception that gelatin is substituted for the copolymer. The emulsion is divided into two portions (identified as F and G) and washed, coated, exposed, heated and photodeveloped. The results are summarized in Table III.

TABLE III Emul- Relative sion Wash method speed 7 rain. Du,-

F Noodle wash (two 100 1.2 .10 .64

hours). G Ion exchange resins 1. 2 .10 69 halide, peptizer and extraneous soluble salts and then removing said extraneous soluble salts, the improvement which comprises washing the silver halide emulsion to a pAg of about 5.5-8.5 by contacting said mixture with anionic and cationic ion-exchange resins which are capable oat substituting hydroxyl ions and hydrogen ions for said extraneous soluble salts and separating the thus washed mixture from the ion-exchange resins.

2. The process of claim 1 wherein the peptizer is gelatin.

3. The process of claim 1 wherein the peptizer is a synthetic polymer.

4. The process of claim 3 wherein the synthetic polymer comprises units of an acrylate ester in which the respective alcohol condensation residue of the ester comprises an organic radical having at least one sulfide-sulfur atom linking two alkyl carbon atoms and units of at least one other polymerized ethylenically unsaturated monomer.

5. The process of claim 3 wherein the synthetic polymer is copoly(3-thiapentylacrylate-3-acryloxypr0pane-1- sulfonic acid sodium salt).

6. A process of claim 1 wherein the mixture is passed through a column containing ion-exchange resins.

7. A process of claim 6 wherein a plurality of ionexchange columns are employed in sequence, such that the eflluent of the preceding column is fed to the succeeding module.

8. A process of claim 6 wherein the column contains a mixture of cationic exchange resins and anionic exchange resins.

9. The process of claim 8 wherein a column containing cationic resin alternates with a column containing an anionic resin.

10. The process of claim 1 wherein the mixture is an aqueous mixture.

References Cited UNITED STATES PATENTS 2,678,885 5/1954 Porter 96114.7 2,827,428 3/ 1958 Noble 9694 3,241,969 3/1966 Hart 96114.7

OTHER REFERENCES West & Todd Textbook of Biochem., 1961, Macmillan, New York, p. 106', 3rd ed., 1961.

I. TRAVIS BROWN, Primary Examiner A. T. SURO PICO, Assistant Examiner US. Cl. X.R. 96114, 114.7 

